As Recently, a variety of large-capacity optical transmission systems have been proposed to meet a rapidly increasing communication demand. Especially, for an access network that occupies the majority of an investment in data transmission network facilities, a low cost and high capacity optical transmission network must be introduced to lower investment costs as well as to meet a communication demand. To meet such requirements, optical access networks having various structures have been researched and reported.
To construct such a low cost optical transmission network, a passive optical transmission network, in which active devices are not included between subscribers and a central office and which includes only passive devices, must be considered. Furthermore, maintenance costs must be reduced by reducing the number of control elements in remote node located between the subscribers and the central office.
Korean Pat. No. 325687 filed by and issued to the present applicant discloses a light source for wavelength division multiplexing optical communication using a Fabry-Perot Laser Diode (FP-LD) wavelength-locked by injected incoherent light. In the Korean Pat. No. 325687, there is presented a light source, in which narrowband incoherent light is externally injected into a FP-LD so that some of the oscillation modes having wavelengths different from that of the injected light are suppressed and the output wavelength of the FP-LD is caused to be locked to a wavelength identical with that of the injected light.
In the FP-LD wavelength-locked by the injected incoherent light, the wavelengths of the light sources of an remote node demultiplexer and subscribers are automatically tuned to each other. Accordingly, the FP-LD wavelength-locked by the injected incoherent light is advantageous in that the temperature control of the devices is not required and sufficient power for the long-distance passive transmission of data is inexpensively achieved, unlike other light sources, so that a Wavelength Division Multiplexing (WDM) passive optical network using such a FP-LD can be inexpensively constructed.
Generally, a WDM Passive Optical Network (PON) introduced to support large-capacity subscribers has a structure as shown in FIG. 1.
Each of the subscribers is connected to an Optical Network Unit (ONU) 13, possesses an optical transmission device having a unique wavelength, and sends a transmission signal, which is modulated to a Non Return-to-Zero (NRZ) digital signal, to a Multiplexer (MUX) 12. The MUX 12 multiplexes signals transmitted from the subscribers using WDM, and transmits a multiplexed signal to a Central Office (CO) 11 over a single optical fiber.
However, a WDM transmission system using a NRZ modulation method per channel generally allows a narrower bandwidth to be used to transmit data compared to the broadband characteristics thereof, so that it is problematic in that it has lower spectral efficiency. To solve the problem, there has been an attempt to improve the spectral efficiency by increasing a transmission rate per channel. However, the above-described method is not suitable for increasing the spectral efficiency in the general optical network because the general optical network has characteristics in which a low transmission rate in a low band and a large number of the channels are required, differently from a general back-bone network having a small number of channels and a high transmission rate. Accordingly, a Subcarrier Division Multiple Access (SCMA) that is capable of dealing with a larger number of subscribers by re-multiplexing each WDM optical channel using Time Division Multiplexing (TDM) or dividing each WDM optical channel using a plurality of Radio Frequency (RF) subcarriers is considered as an alternative to the above-described method.
FIGS. 2 and 3 are configuration diagrams showing a general TDMA PON and an SCMA PON, respectively.
In FIG. 2, a CO 21 allocates time slots and transmits frames, and ONUs 23 receives the frames in the time slots allocated to themselves through a splitter 22.
Meanwhile, in FIG. 3, a CO 31 and an ONU 32 are connected to each other via a single optical fiber, and the ONU 32 is provided with a frequency combiner 33, so that the CO 31 communicates with each subscriber 34 through a subcarrier frequency.
FIG. 4 is a configuration diagram of a combined TDMA and WDM PON, and FIG. 5 is configuration diagram of a combined SCMA and WDM PON.
In the PON of FIG. 4, a CO 41 is connected to a MUX 42 via a single optical fiber, and a splitter 43 is connected to each of the ports of the MUX 42. Additionally, the plural number of ONUs 44 are connected to the splitter 43. Accordingly, an optical signal transmitted from the CO 41 is divided according to wavelengths, and the splitter 43 divides the divided optical signal according to time slots and provides divided optical signals to the ONUs 44.
In FIG. 5, a CO 51 and a wavelength division MUX 52 are connected to each other via a single optical fiber, and an ONU 53 is connected to each of the ports of the MUX 52. An optical signal transmitted from the CO 51 is divided by the MUX 52, and the ONU 53 divides a divided optical signal according to frequencies through a combiner 54 and communicates with subscribers 55.
FIG. 6 is a configuration diagram of a PON in which the wavelength of light sources located at a subscriber is shared to increase the access efficiency of the SCMA-WDM access network.
Wavelengths divided by a MUX located between a CO 61 and subscribers 66 are shared using splitters 63 located next to the MUX 62. Lights modulated onto subcarriers carrying subscriber data are combined together in a single optical fiber in the splitters 63 used as a combiner, and wavelength groups are multiplexed using WDM, and transmitted to the CO 61. When the data is received, a Signal-to-Noise Ratio (SNR) is important when a wavelength is shared, the light shared in each of the wavelength groups interfere with each other, so that Optical Beat Interference (OBI) noise is generated. As a line width is narrowed and the centers of wavelengths coincide with each other, the OBI noise is increased, so that the OBI noise becomes dominant than the thermal noise or the shot noise. Meanwhile, the SNR of data received from the CO 61 is significantly influenced by the OBI noise. Accordingly, with a general Distributed Feedback Laser Diode (DFB-LD) or a FP-LD wavelength-locked by injected coherent light, it is difficult to construct an access network using the method of sharing a wavelength.
The TDMA-WDM PON of FIG. 4 and the SCMA-WDM PON of FIG. 6 have a common feature in that they share wavelengths. This wavelength sharing-type network may be considered to expand the number of subscribers. In this case, DFB-LDs, Light Emitting Diodes (LED) or FP-LDs can be used as light sources used at subscriber ends. The DFB-LDs have a high side mode suppression ratio and high output power. However, the DFD-LDs have a narrow line width, so that high OBI noise is generated in the wavelength sharing-type construction. Furthermore, manufacturing costs are relatively high, so that the high manufacturing costs act as a disadvantage in implementing a low cost access network. Accordingly, inexpensive diodes, such as the LEDs or the FP-LDs, have been used to construct the convectional PON in view of the manufacturing costs. However, the LEDs are disadvantageous in that the power budget design of an optical transmission link is seriously limited due to the low output power thereof, and the FP-LDs are disadvantageous in that transmission quality is degraded due to mode-partition noise generated by filtering.
To overcome these disadvantages, there has been proposed a technique of filtering the Amplified Spontaneous Emission (ASE) of an optical fiber amplifier in a spectral region and employing filtered ASE. The technique overcomes disadvantages, such as the low-power output of the LED or the mode-partition noise of the FP-LD, but still has the disadvantage of high manufacturing costs because a direct modulation is impossible.